In common control type switching systems, call billing data functions are often performed by apparatus integral with the call processing equipment. This arrangement is advantageous since the calling line and called line identities and the supervisory states used in establishing the talking connection can also be used to record and time billing entries.
In other types of switching systems, such as the direct progressive step-by-step system, data accumulators are appliqued to the telephone switching system to identify the calling and called lines and to monitor the talking connection to ascertain answer and disconnect times. One example of such a system is disclosed in U.S. Pat. No. 3,967,074 to R. F. Schillo of June 29, 1976.
The Schillo patent discloses a Call Data Accumulator (CDA) which monitors the tip, ring and sleeve conductors between the line finder and first selector in a step-by-step system. The CDA detects an initial seizure by the calling customer, records the called number dialed by the calling customer, and monitors the connection for answer and disconnect supervisory signals in order to time the call. The identity of the calling line is derived by the CDA in cooperation with Automatic Number Identification (ANI) equipment. Typically, the ANI equipment momentarily actuates a sleeve relay to connect a tone over an "M" lead to the sleeve lead of the connection at the line finder. The tone is transmitted back over the sleeve lead to the calling customer line equipment and to a translator which converts the line equipment number to the directory number which is to be billed. Depending on the size of the telephone switching office, the office may be served by one or more CDA billing subsystems.
Due to the multipling of "M" leads and the use of common translators within a CDA subsystem, only one calling line can be identified at a time to avoid billing errors. In the event a fault occurs, such as a "held" sleeve lead relay, the CDA subsystem is incapable of billing other calls which originate from lines served by the same CDA.